The present invention relates to a novel polyester coating composition characterized by having a lower solvent requirement for attaining a coating viscosity, improved physical properties, and a better cure response than polyester resin compositions currently available.
Oil-modified alkyd (polyester) resins have long been used in the paint industry, and generally are prepared by the reaction of polybasic organic acids, polyhydric alcohols and unsaturated fatty acids. The "oil length" of an oil modified alkyd resin may be defined as the weight percent of the resin contributed by the aliphatic, "fatty" chains of the fatty acids. As pointed out in U.S. Pat. No. 4,111,871 (Aritomi), the oil length of an alkyd resin is important to the physical properties of the paint or coating which the resin forms. Oil length affects solubility, hardness, gloss, color preserving ability, weather resistance, hardening time and storage life of the paints made from alkyd resins. Increasing the oil length is reported in U.S. Pat. No. 4,111,871 to enhance the flexibility of the resulting film, to increase its solubility in organic solvents, and to decrease film hardness. Reduction in oil length is said to enhance gloss and color preserving ability of the film.
The above-mentioned U.S. Pat. No. 4,111,871 describes a resin composition which comprises an oil-modified alkyd resin and an acrylate or methacrylate monoester of an alcohol. The resulting resin composition is a liquid and is said to be usable without the addition of solvent. Of importance, the oil length of the oil-modified alkyd resin must necessarily be at least 40% to avoid inferior waterproofness of the initial film coating. Aritomi U.S. Pat. No. 4,147,675 shows the use of oil-modified alkyd resins when combined with sorbic, crotonic or 2(beta-furyl) acrylic acids.
U.S. Pat. Nos. 4,224,202 and 4,225,473 (Heiberger) refer to high-solids coating compositions containing an unsaturated fatty acid alkyd resin, an alkyl dimethacrylate or trimethacrylate monomer, a protected cobalt oxime catalyst, a polar solvent and a peroxide. The oil length of the alkyd resin disclosed in these references exceeds 40%.
U.S. Pat. No. 4,014,830 (Rumfield) discusses an acrylate or methacrylate modified alkyd resin or epoxy ester which contains a small amount (up to 10% by weight) of an acrylate or methacrylate ester of a polyol and which is characterized by rapid drying.
Commercially available alkyd resin coating compositions which have excellent physical properties and cure responses commonly are quite viscous and accordingly require, for proper coating, a substantial amount of solvent such as methyl propyl ketone, xylene or butyl acetate, and such resin compositions commonly have volatile organic contents ("VOC"), primarily due to organic solvents, of 3.5 lbs./gallon (420 g/l) and above. Alkyd coating compositions which require less solvent to attain a coating viscosity generally suffer from poorer physical properties and poorer cure responses.
Conventional and high solids industrial alkyd resins may have oil lengths on the order of 20 to 50% and commonly must be dissolved in aromatic and oxygenated solvents to be used as air dried paints or oven cured paints.
Difficulty has been experienced in achieving high solids coatings that have the low viscosities necessary for industrial applications. A major factor involves the molecular weight of the alkyd resin. Generally speaking, increasing the molecular weight of an alkyd resin generally improves physical characteristics of a coating made from the resin and decreases the coating drying time; that is, the time required for the coating to dry sufficiently so that it can be handled without damage. However, increasing the molecular weight also increases the amount of solvent needed to achieve a viscosity appropriate for coating applications. The amount of solvent that is needed to provide the desired coating viscosity may be reduced by increasing the oil length of the alkyd. However increasing the oil length increases the drying time.
It would be highly desirable to prepare alkyd resin coating compositions which have relatively low solvent concentrations and which yet exhibit good coating properties and excellent physical and cure response properties.